The present invention relates to a process for the production of chlorine dioxide from an alkali metal chlorate, a mineral acid and hydrogen peroxide as the reducing agent. The invention also relates to a process for the production of chlorine dioxide from chloric acid or a mixture of chloric acid and alkali metal chlorate. More particularly, the invention relates to production of chlorine dioxide in the presence of a complexing agent. Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
2. Description of the Related Art
There are a lot of different processes for chlorine dioxide production. The most commonly used reducing agents are chloride ions, methanol and sulphur dioxide. The draw back with chloride ions as the reducing agent is the formation of half a mole of chlorine for each mole of chlorine dioxide formed. Increasing environmental demands have brought about a change over to reducing agents which do not produce chlorine as a by-product, mainly methanol. A draw back with methanol as the reducing agent can be the formation of chlorinated organic compounds, from by-products of methanol, in the bleaching train. It is well known that the efficiency of the added methanol is lowered due to side reactions where formaldehyde and formic acid are formed. Also some of the methanol leaves the reactor without having participated in the reduction. The corresponding ether and ester are probably there as well. It could be expected that reactions can occur in the bleaching train with the aldehyde, acid, ether and ester resulting in chlorinated organic compounds. The reduction with methanol is also rather slow at low acid strength in the interval 2-5N if catalysts are not incorporated.
In U.S. Pat. Nos. 5,091,167 and 5,091,166 it was found that hydrogen peroxide was a surprisingly effective reducing agent in the whole acid interval 2-11N. It was found that with hydrogen peroxide as the reducing agent it is possible to obtain a process with a very high reaction rate and efficiency and in the interval 2-5N for the acid strength, the reaction rate exceeded the rate of known processes with several hundreds percent without the aid of catalysts. It was also possible to produce essentially chlorine free chlorine dioxide. The following formula illustrate the reaction for acidities of 2-5N: EQU 2 NaClO.sub.3 +H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 .fwdarw.2 ClO.sub.2 +Na.sub.2 SO.sub.4 +2 H.sub.2 O +O.sub.2 EQU and EQU 6 NaClO.sub.3 +4 H.sub.2 SO.sub.4 +3H.sub.2 O.sub.2 .fwdarw.6 ClO.sub.2 +2 Na.sub.3 H(SO.sub.4).sub.2 +6 H.sub.2 O +3O.sub.2
for acidities of 5-11N.